Non-destructive investigation of the Ag distrubution in the late roman folles from about 300 to 333 AD

The knowledge of silver content in late roman folles (from about 300 to 330 AD) has been the object of intensive investigation. The non destructive NAA has shown itself to be the most appropriate technique to investigate silver overall content with good accuracy. The XRF technique is not indicated to determine the overall body content because of the expected dishomogeneity of the silver alloy. A systematic investigation has evidenced strong variations in Ag content in coins of different period of coinage. They have been interpreted as an indication of an inflation process. Also evidence of thin silver enriched patina have been found. However no systematic investigation has been carried out on the patina characterisation. In the present work we propose a method based on the use of a PIXE portable system in order to evaluate the patina composition and, in combination with a portable XRF technique, to estimate its thickness. Both portable systems were realised at the INFN/LNS laboratory. The method was tested on reference samples consisting of a silver enriched patina of different thickness and on few roman folles. In the near future it will be applied to a systematic study of the known "Misurata Treasure" (Lybia).